This invention relates to electrochemical fuel cells. More particularly, this invention relates to electrochemical fuel cells which employ hydrogen as a fuel and receive an oxidant to convert the hydrogen to electricity and heat, and which utilize a proton exchange membrane as the electrolyte.
Generally, a fuel cell is a device which converts the energy of a chemical reaction into electricity. It differs from a battery in that the fuel cell can generate power as long as the fuel and oxidant are supplied.
A fuel cell produces an electromotive force by bringing the fuel and oxidant into contact with two suitable electrodes and an electrolyte. A fuel, such as hydrogen gas, for example, is introduced at a first electrode where it reacts electrochemically in the presence of the electrolyte and catalyst to produce electrons and cations in the first electrode. The electrons are circulated from the first electrode to a second electrode through an electrical circuit connected between the electrodes. Cations pass through the electrolyte to the second electrode. Simultaneously, an oxidant, typically air, oxygen enriched air or oxygen, is introduced to the second electrode where the oxidant reacts electrochemically in presence of the electrolyte and catalyst, producing anions and consuming the electrons circulated through the electrical circuit; the cations are consumed at the second electrode. The anions formed at the second electrode or cathode react with the cations to form a reaction product. The first electrode or anode may alternatively be referred to as a fuel or oxidizing electrode, and the second electrode may alternatively be referred to as an oxidant or reducing electrode. The half-cell reactions at the two electrodes are, respectively, as follows:
H2xe2x86x922H++2exe2x88x92
1/2O2+2H++2exe2x88x92xe2x86x92H2O
The external electrical circuit withdraws electrical current and thus receives electrical power from the cell. The overall fuel cell reaction produces electrical energy which is the sum of the separate half-cell reactions written above. Water and heat are typical by-products of the reaction.
In practice, fuel cells are not operated as single units. Rather, fuel cells are connected in series, stacked one on top of the other, or placed side by side. A series of fuel cells, referred to as fuel cell stack, is normally enclosed in a housing. The fuel and oxidant are directed through manifolds to the electrodes, while cooling is provided either by the reactants or by a cooling medium. Also within the stack are current collectors, cell-to-cell seals and insulation, with required piping and instrumentation provided externally of the fuel cell stack. The stack, housing, and associated hardware make up the fuel cell module.
Fuel cells may be classified by the type of electrolyte, either liquid or solid. The present invention is primarily concerned with fuel cells using a solid electrolyte, such as a proton exchange membrane (PEM). The PEM has to be kept moist with water because the available membranes will not operate efficiently when dry. Consequently, the membrane requires constant humidification during the operation of the fuel cell, normally by adding water to the reactant gases, usually hydrogen and air.
The proton exchange membrane used in a solid polymer fuel cell acts as the electrolyte as well as a barrier for preventing the mixing of the reactant gases. An example of a suitable membrane is a copolymeric perfluorocarbon material containing basic units of a fluorinated carbon chain and sulphonic acid groups. There may be variations in the molecular configurations of this membrane. Excellent performances are obtained using these membranes if the fuel cells are operated under fully hydrated, essentially water-saturated conditions. As such, the membrane must be continuously humidified, but at the same time the membrane must not be over humidified or flooded as this degrades performances. Furthermore, the temperature of the fuel cell stack must be kept above freezing in order to prevent freezing of the stack.
Cooling, humidification and pressurization requirements increase the cost and complexity of the fuel cell, reducing its commercial appeal as an alternative energy supply in many applications. Accordingly, advances in fuel cell research are enabling fuel cells to operate without reactant conditioning, and under air-breathing atmospheric conditions while maintaining usable power output.
The current state-of-the-art in fuel cells, although increasingly focusing on simplified air-breathing, atmospheric designs, has not adequately addressed operations in sub-zero temperatures, which requires further complexity of the design. For instance, heat exchangers and thermal insulation are required, as are additional control protocols for startup, shut-down, and reactant humidifiers.
Where a solid polymer proton exchange membrane (PEM) is employed, this is generally disposed between two electrodes formed of porous, electrically conductive material. The electrodes are generally impregnated or coated with a hydrophobic polymer such as polytetrafluoroethylene. A catalyst is provided at each membrane/electrode interface, to catalyze the desired electrochemical reaction, with a finely divided catalyst typically being employed. The membrane electrode assembly is mounted between two electrically conductive plates, each of which has at least one flow passage formed therein. The fluid flow conductive fuel plates are typically formed of graphite. The flow passages direct the fuel and oxidant to the respective electrodes, namely the anode on the fuel side and the cathode on the oxidant side. The electrodes are electrically coupled in an electric circuit, to provide a path for conducting electrons between the electrodes. In a manner that is conventional, electrical switching equipment and the like can be provided in the electric circuit. The fuel commonly used for such fuel cells is hydrogen, or hydrogen rich reformate from other fuels (xe2x80x9creformatexe2x80x9d refers to a fuel derived by reforming a hydrocarbon fuel into a gaseous fuel comprising hydrogen and other gases). The oxidant on the cathode side can be provided from a variety of sources. For some applications, it is desirable to provide pure oxygen, in order to make a more compact fuel cell, reduce the size of flow passages, etc. However, it is common to provide air as the oxidant, as this is readily available and does not require any separate or bottled gas supply. Moreover, where space limitations are not an issue, e.g. stationary applications and the like, it is convenient to provide air at atmospheric pressure. In such cases, it is common to simply provide channels through the stack of fuel cell for flow of air as the oxidant, thereby greatly simplifying the overall structure of the fuel cell assembly. Rather than having to provide a separate circuit for oxidant, the fuel cell stack can be arranged simply to provide a vent, and possibly, some fan or the like to enhance air flow.
There are various applications for which humidification of fuel cells poses particular problems and challenges. For example, operation of fuel cells in mobile vehicles usually means that there is no readily available supply of water for humidifying incoming oxidant and fuel streams. It is usually undesirable to have to provide water to a vehicle for this purpose and also to have to carry the excess weight of the water around in the vehicle. In contrast, for stationary applications, providing a supply of water for humidification is usually quite possible.
However, there also some stationary applications for which humidification is not straightforward. For example, fuel cells are often used to provide power supplies to remote sensing equipment, in locations where water may not be readily available. Additionally, such remote use of fuel cells often occurs at locations with extreme climatic conditions. Thus, it has been known to use fuel cell stacks in the Antarctic regions and the like, for providing supply to scientific instruments. It is simply not realistic to provide a separate supply of water for humidification, because of the problems of preventing freezing of the water supply. Additionally, ambient air used as an oxidant is excessively dry, so that humidification is more critical than when using relatively moist air at more moderate temperatures. It will be appreciated that similar extreme conditions can be found in desert locations and the like.
PEM fuel cells commonly operate at an elevated temperature. However, a common problem with many fuel cell stacks is that it is difficult to maintain uniform operating conditions throughout the stack. More particularly, central cells in the stack can be maintained at uniform conditions, but cells at either end of the stack tend to operate at less than optimum conditions. As a result, frequently the end cells have a tendency to become flooded, and as a consequence to have decreased efficiency and performance.
A further consideration is that, for many applications, it is desirable that an overall fuel cell power installation be compact and include a minimum number of components. Thus, the inclusion of a separate humidification section or unit is, in general, undesirable, as it leads to additional complexity and a further component to be provided in the overall assembly.
What the present inventors have realized is that the problems identified humidification sections integral with a fuel cell stack and having a construction broadly similar to that of the fuel cell stack itself.
In Proton Exchange Membrane (PEM) fuel cells, both the fuel and oxidant streams need to be humidified in order to enhance the fuel cell performance. For good performance, the membrane needs to be kept at a desired level of humidity. At the same time, the electrochemical reaction between the fuel and the oxidant generates water during power generation, and the resultant water vapor needs to be discharged from the cell. Because of the nature of the PEM fuel cells, the production water is largely entrained in the discharged oxidant flow.
The present inventors have realized that it is possible to recover moisture from the discharged oxidant stream, by passing it across one side of a membrane, with a fuel stream passing on the other side of the membrane. Similarly, the discharged fuel stream can include some moisture and could be passed on one side of another membrane, with the incoming oxidant stream passing on the other side of that other membrane. As the inlet fuel and oxidant streams are dry and the outgoing fuel and oxidant streams are wet or humid, this enables moisture to transfer from the outgoing streams to the ingoing streams.
Thus, humidity or moisture is transferred from the outgoing or discharged oxidant stream through a polymer membrane or the like to a dry, counter-flowing incoming fuel stream. Simultaneously, the outgoing, humid fuel stream transfers water through another membrane to the incoming, dry oxidant stream.
The two membranes are provided in respective cells, or stacks of cells, identified as a fuel humidification cell stack and an oxidant humidification cell stack. It is also proposed to provide the fuel humidification cell stack and the oxidant humidification cell stack at either end of the fuel cell stack. As noted, it has been realized that a common problem with many fuel cell stacks is that cells at or near the end of the fuel cell stack often function at poor efficiency and often become flooded with water. This is because they do not operate at a sufficiently high temperature to cause excess water to be discharged as water vapor.
The inventors have realized that by mounting the humidification cell stacks at either end of the convention fuel cell stack, this can help stabilize the temperature of the entire fuel cell stack, and in effect ensure that all of the individual fuel cells are operating at substantially the same temperature, so as to overcome this problem. Additionally, by providing humidification cells or cell stacks on both ends of the power generation cell stack, the water is recovered before it hits the power generation cells.